Electron discharge devices and to circuit arrangements embodying such devices



ELECTRON nIsHAcE DEvIcEs AND TO CIRCUIT ARRANGEMENTS EMBODYING SUCH DEVICES Filed Feb. :5, 1958 Nov- 15, 196 J A LODGE ETAL 2,960,617

'4 4 i L i 1 2 l +ve v FIG.1

United States Patent ELECTRON DISCHARGE DEVICES AND TO CIR- gIIg' IICfiASRRANGEMENTS EMBODYING SUCH James Alec Lodge, Sunbury on Thames, and Reginald Sidney Webley, Hayes, England, assignors to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Filed Feb. 3, 1958, Ser. No. 712,859

Claims priority, application Great Britain Feb. 7, 1957 8 Claims. (Cl. 313-89) This invention relates to electron discharge devices and to circuit arrangements embodying such devices.

Various proposals have heretofore been made to employ in electron discharge devices such as are suitable for television and other purposes a target electrode which includes a solid layer of material which is normally a non-conductor or a semi-conductor but which when bombarded with a high velocity beam of electrons becomes conducting, this property being termed induced conductivity. Several devices employing target electrodes exhibiting such properties are disclosed in the specification of British Patent No. 668,727. It is found that target electrodes embodying solid layers of material which exhibit the property of induced conductivity possess an undesirable degree of lag. For example in one of the electron discharge devices disclosed in the specification of the aforesaid patent there is projected onto one side of the target electrode an electron image of a sulficiently high velocity to cause induced conductivity and the opposite side of the target electrode is scanned by a low velocity electron beam so that the elemental charges set up on the target electrode by the high velocity electron image can be restored to a substantially equilibrium potential, such restoration generating picture signals suitable for television. The lag aforesaid is manifest in that an appreciable time elapses before the charge on the surface of the target electrode is restored to its equilibrium value which is an undesirable feature. For example it is found that with a solid layer having a thickness of 0.5 1. the lag had a duration of several seconds. It has heretofore been considered necessary to make the layer of material thin say of the thickness aforesaid, in order to cause it to exhibit the desired effect of induced conductivity at a suitable working voltage. The lag aforesaid may be caused as the result of inherent lag in the material itself and/ or due to the fact that the layer is so thin that the capacity across the layer is necessarily high.

The object of the present invention is to provide an improved electron discharge device embodying a layer which exhibits the property of induced conductivity when bombarded with high velocity electrons, with a view to reducing lag.

In one aspect of the present invention instead of using a solid layer which has the property of bombardment induced conductivity a spongy layer of said material is employed. Such a spongy layer is defined as a porous layer composed of loosely packed particles of said material. 7

-According therefore to one feature of the invention there is' provided an electron discharge device comprising a target including a spongy layer of material which has the property of bombardment induced conductivity, means whereby said layer can be bombarded with electrons having such a high velocity as to cause said layer to'exhibit induced conductivity whereby said layer acquires different charges dependent on the intensity of the electrons which bombard said layer, and means whereby lower velocity electrons can be projected onto said layer so as to change said charges.

According to another feature of the invention there is provided a circuit arrangement including an electron discharge device according to the preceding paragraph having means for accelerating electrons to a sufficiently high velocity to cause said layer to exhibit induced conductivity and means whereby said low velocity electrons are caused to be projected onto said layer.

According to another feature of the invention there is provided an electron discharge device comprising a cathode, a control electrode for controlling an electron stream from said cathode, a target comprising a conductor having deposited thereon a spongy layer of a material which has the property of bombardment induced conductivity and a further conductor on the side of said layer of material remote from said first mentioned conductor. Such a discharge device is similar to that shown in Figure 5 of the specification of British Patent No. 668,727.

According to a further feature of the invention there is provided a circuit arrangement including an electron discharge device according to the preceding paragraph having means for accelerating electrons from said cathode to a substantially high velocity to cause said electrons to be projected onto said layer to cause it to exhibit induced conductivity.

Another feature of the invention consists in employing antimony trisulphide as a bombardment induced conductivity target.

The present invention can be applied to any of the forms of electron discharge devices disclosed in the specification of the aforesaid patent and can also be applied in general to any electron discharge device in which the property of induced conductivity is required to be employed.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings, in which: 1

Figure 1 illustrates the invention as applied by way of example to the television pick-up tube shown in Figure 3 of the specification of British Patent No. 668,727, and

Figure 2 illustrates in cross section on an enlarged scale the target employed in the device shown in Figure 1 and embodying a modification.

As shown in Figure 1 the device comprises an evacuated envelope 1 in which is arranged a target in accordance with the invention. The target comprises a conducting grid 2 which may have for example 600 interstices per linear inch on which is disposed a thin continuous sheet of conducting material such as aluminum which serves to support a continuous layer 4 of a material which can exhibit induced conductivity under electron bombardment. The sheet 3 of conducting material may be formed by any known manner as by first providing on the surface of the grid 2 a film of collodion after which aluminum is evaporated onto the collection film, the collodion film being then removed by heating. At one end of the envelope 1 there is provided a photoelectric cathode 5 in contact with a transparent conducting electrode 6 and adapted to receive an optical image of a subject for transmission from an optical lens system as indicated conventionally. When an optical image is projected on to the photo-electric cathode 5 photo-electrons are released and are accelerated towards the target electrode by an electrode 7 and are focussed by means of a focussing coil 8. In the example shown in Figure l the photo-electrons are projected through the grid 2, through the thin continuous sheet of conducting material 3 and then on to the layer 4. The surface of the layer 4 remote from the photo-electric cathode 5 is adapted to be scanned by a low velocity electron beam generated from an electron gun comprising a thermionic cathode 9, a cathode screen '10 and accelerating electrode 11. The electron beam is further accelerated by a wall anode 12 and is then decelerated by a decelerating electrode 13 and is scanned over the layer 4 by means of scanning coils 14. The photo-electric image from the photo-electric cathode 5 is accelerated to a suitable high velocity by maintaining the electrode 6 at a suitable negative potential and the electrode 7 at a suitable positive potential in relation to the potential of the sheet 3 the accelerated electrons passing through the latter and enter into the layer 4 so that elemental areas of the layer 4 acquire different conductivities dependent on the intensity of corresponding areas of the electron image which in turn are dependent on the illumination of corresponding elemental areas of the optical image. This results in a charge pattern being built up on the surface of the layer 4 facing the electron gun, and current greater than the bombarding current of the electron image flowing through the layer 4 so that current amplification is obtained. Such surface of the layer 4 is then scanned by the electron beam from said gun restoring said elemental areas to an equilibrium potential. The beam from the cathode 9 is scanned over the surface of the target in the presence of the longitudinal magnetic field generated by a solenoid coil 15 so that the scanning beam impinges on the target electrode substantially normal throughout the scanning movement. The equilibrium potential of the target electrode corresponds substantially to that of the potential of the cathode 9 and in this case the conducting sheet 3 is maintained at a positive potential of say 100 volts with respect to the cathode 9 so that a positive charge image will be set up on the scanned surface of the layer 4, this positive charge image being then discharged by the low velocity scanning beam, discharge of the elemental areas of the scanned surface setting up picture signals in a resistance 16 connected to the conducting sheet 3. The required potentials for application to the electrodes 9, 10, 1'1, 12 and 13 may be derived from a source 17 across which a potentiometer 18 is arranged, the electrodes being connected to various tappings on the potentiometer whereby the cathode 9 is maintained at earth potential, the cathode shield 10 at a negative potential with respect thereto, the accelerating electrode 11 at a positive potential with respect to cathode 9, the wall anode 12 at a high positive potential with respect thereto and the decelerating electrode 13 at a lower positive potential with respect to the wall anode 12.

In the present invention the layer 4 is of a spongy form, that is to say it is a porous layer composed of loosely packed particles. One suitable material from which the layer 4 can be formed is red antimony trisulphide, and in order to form such a spongy layer the antimony trisulphide is evaporated onto the conducting sheet 3 by evaporating said material in a low gas pressure. The gas may be air, argon or xenon or any other suitable gas. The gas pressure may be 0.3 mm. of mercury when employing xenon for example although higher gas pressures may be employed for example as high as 1 mm. or more of mercury. The mean free path of the evaporated particles in the gas should be appreciably less than the distance over which evaporation is carried out. For example the mean free path may be 0.01 cm. and the evaporating source may be disposed at a distance of 1 cm. from the target to form a layer 1 cm. square. The spongy layer may have a thickness greater than 0.5a and it is found that with a spongy layer of 2-3 in thickness the lag is about /5 of a second or less.

It may be found that the gain of a spongy layer of the same thickness as a solid layer may be less than is the case with a solid layer. However, with a solid layer of say 0.5a the velocity of the electron stream required to produce induced conductivity with maximum gain may be required to be as high as 12 kilovolts and with a thickness of 2 the velocity may be as high as 24 kilo-. volts. With a spongy layer of 0.5;1. in thickness a maximum gain occurs with an electron velocity of three kilovolts but the gain is very low. With a spongy antimony trisulphide layer of 2 in thickness an electron velocity of 7 kilovolts is found to produce a gain of about 10. By employing a spongy layer the permittivity of the layer is reduced with the result that for the same layer thickness the capacity across the layer is reduced compared with that of a solid layer. Thus merely re placing a solid layer by a spongy layer of the same thickness the capacity across the layer is reduced with the result that lag due to capacity is likewise reduced. It has been found that with a spongy layer of antimony trisulphide of 2 in thickness the total lag does not exceed the capacitive lag.

Instead of employing antimony trisulphite for the layer other material such as arsenic trisulphide, silicon monoxide, silico-n dioxide, magnesium fluoride, calcium fluoride and zinc sulphide are also suitable and can also be deposited in spongy form. If desired as shown in Figure 2 after deposition of the spongy layer 4 a thinner solid layer 20 of material exhibiting the property of bombardment induced conductivity may be deposited on the layer 4. The solid layer 20 is arranged to face the scanning beam. The solid layer may be formed by evaporation of the material in vacuum. Also if desired prior to the deposition of the spongy layer 4 a solid layer may be first deposited on the conducting sheet 3 and if desired alternatively or in addition to said one or both additional solid layers, a solid layer may be employed having spongy layers on both sides thereof. The solid layer or layers may be very thin and may be about 0.1 1. in thickness and may be of the same material as the layer 4 or may be of a different material or materials.

What we claim is:

1. An electron discharge device comprising a target including a spongy layer of material of the nature obtained by evaporation in a gaseous atmosphere and which has the property of bombardment induced conductivity, means whereby said layer can be bombarded with electrons having such a high velocity as to cause said layer to exhibit induced conductivity whereby said layer acquires different charges dependent on the intensity of the electrons which bombard said layer, and means whereby lower velocity electrons can be projected onto said layer so as to change said charges.

2.. An electron discharge device comprising a cathode, a control electrode for controlling an electron stream from said cathode, a target comprising a conductor having deposited thereon a spongy layer of a material of the nature obtained by evaporation in a gaseous atmosphere and which has the property of bombardment induced conductivity and a further conductor on the side of said layer of material remote from said first mentioned conductor.

3. An electron discharge device according to claim 1 wherein said layer is composed of red antimony trisulphide.

4. An electron discharge device according to claim 3 wherein said spongy layer has a thickness of 23,u.

5. An electron discharge device according to claim 1, wherein said target includes at least one solid layer of material exhibiting the property of bombardment induced conductivity which is thinner than said spongy layer.

6. An electron discharge device having a target of the bombardment induced conductivity type wherein Said target includes a layer of antimony trisulphide of the nature obtained by evaporation in a gaseous atmosphere.

7. A circuit arrangement including an electron discharge device according to claim 1, having means for accelerating the electrons to a sufiiciently high velocity onto said layer to cause said layer to exhibit induced conductivity and means whereby said low velocity electrons are caused to be projected onto said layer.

8. A circuit arrangement including an electron discharge device according to claim 2, having means for accelerating electrons from said cathode to a substantially 6 high velocity to cause said electrons to be projected onto said layer to cause it to exhibit induced conductivity.

References Cited in the file of this patent UNITED STATES PATENTS 2,544,755 Johnson Mar. 13, 1951 2,645,721 Williams July 14, 1953 2,683,832 Edwards et al. July 13, 1954 2,740,837 Kirkpatrick Apr. 3, 1956 2,776,371 Clogston et a1. Jan. 1, 1957 

